1. Field of the Invention
The present invention relates generally to nuclear medical imaging devices and more particularly relates to Single Photon Emission Computed Tomography (SPECT) nuclear medicine studies and correction of data attenuation in such studies.
2. Introduction
In various environments, such as in medical environments, imaging devices can include detectors that detect electromagnetic radiation emitted from radioactive isotopes or the like within a patient. The detectors typically include a sheet of scintillation crystal material that interacts with gamma rays emitted by the isotope to produce photons in the visible light spectrum known as “events.” The scintillation camera includes one or more photodetectors such as an array of photomultiplier tubes, which detect the intensity and location of the events and accumulate this data to acquire clinically significant images that are rendered on a computer display for analysis.
In a conventional SPECT study of an organ such as the heart, a radioisotope (Tc-99m, Tl-201, for example) is administered to the patient and the radioisotope is taken up by the heart muscles. Then, the patient is placed in an imaging bed of a scintillation camera system and one or more scintillation camera detectors are rotated about the long axis of the patient and interact with gamma emissions from the patient's body at various angular orientations about the axis. The resulting data is used to form three-dimensional images (known as “SPECT images” or “tomographic images”) of the distribution of the radioisotope within the patient.
Such three-dimensional SPECT images can be calculated based on a set of two-dimensional images (“projections” or “projection images”) acquired by the scintillation camera system as the detectors are rotated about the patient in a series of steps; this calculation process is known as image reconstruction. The most commonly employed method of image reconstruction is known as filtered back-projection or FBP. When FBP reconstruction is used to reconstruct SPECT images from two-dimensional projection images obtained from a scintillation camera, some well-recognized distortions introduce errors or artifacts in the result. One of the most critical distortions is caused by attenuation of gamma radiation in tissue.
As a consequence of attenuation, quantitative image values in the various projections do not accurately represent line integrals of the radioisotope distribution within the body. It is therefore necessary to correct for this distortion, and the process for doing so in SPECT is known as attenuation correction.
Many prior art techniques for attenuation correction in SPECT have assumed that the linear attenuation coefficient of the body is uniform and impose such uniformity as a mathematical constraint in the image reconstruction process. However, for a very important class of studies, namely cardiac SPECT studies, the linear attenuation coefficient of the body is in fact highly non-uniform. This is because lung tissue has a lower attenuation than do, e.g., the blood and other non-lung tissue. Further, linear attenuation coefficients may be different for different areas of the body having varying mass, density, etc.
Thus, in SPECT studies of, e.g., the heart, a SPECT reconstruction of the image of radioactivity within the heart will necessarily contain artifacts caused by the unequal attenuation coefficients of, e.g., the lungs and other parts of the body.
It is known to measure the actual attenuation coefficients of body tissues by placing a line source of gamma radiation on one side of the body and measuring the transmission of the gamma radiation through the body as a function of direction, i.e. collecting transmission CT data, as the line source is scanned across the patient's body. See, e.g. U.S. Pat. No. 5,576,545 (Stoub et al.) incorporated herein by reference in its entirety.
However, present methods suffer from certain disadvantages. In particular, FBP does not optimally process the noise or distortion in the projection data. FBP is not statistically based, and the conventional FBP computational algorithm is prone to “streak” artifacts predominantly oriented in the radial direction. The streak artifact significantly degrades the attenuation correction of SPECT images reconstructed from attenuation maps (“μ-maps”) with FBP.
Another problem with existing attenuation correction methods involves the correction of transmission CT data for downscatter by subtracting estimated downscatter values from the transmission data. Attenuation of the transmission radiation beam through a patient can be large (˜50), resulting in count-starved data. Subtraction from this data of estimated downscatter obtained from an adjacent energy comparison window can result in a measurement of zero or even non-physically possible “negative” values. Consequently, use of FBP for transmission reconstruction requires either truncation of downscatter-corrected transmission data to avoid negative values, or use of some other ad-hoc process to fill data “holes.”
Thus, while a variety of methods and apparatus are known as described above, there remains a need in the art for improved methods and apparatus overcoming the above and/or other problems.